Santa Monica was a city on the planet Earth.
Santa Monica was represented by a natural shoreline with a small sailboat on a bag promoting landmarks and locations in California. (Star Trek IV: The Voyage Home)
Captain Janeway and Tuvok explored this city as well as the neighboring city of Los Angeles while searching for the timeship Aeon commander, Braxton. (VOY: "Future's End")
Santa Monica was not explicitly identified in this episode (which instead talked about the the area as part of Los Angeles), but the location visited was very recognizable as being the Santa Monica Pier.